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== Files, descriptors and inodes in Linux ==
== Files, descriptors and inodes in Linux ==
When a task opens a file Linux kernel constructs a chain of 3 (well, 3.5) object to serve this.
When a task opens a file Linux kernel constructs a chain of 3 objects to serve this.
; Inode
; Inode
+------+ +--------+ +-------+
+------+ +--------+ +-------+
</pre>
</pre>
The File object can be referenced by more than on FDT, e.g. when a task calls <code>fork()</code> the child one gets new FDT, but it references the same Files as the parent does, i.e. Files become shared objects.
The Inode object is also interesting. First of all, remember that in Linux file descriptors can be obtained not only by the <code>open()</code> system call, but also by <code>pipe()</code> and <code>socket()</code> and a bunch of Linux-specific <code>epoll_create</code>, <code>signalfd</code> and others. So when serving ''this'' Linux would anyway create the mentioned above chain of File-Dentry-Inode objects, but the Inode one will be different for different calls. And CRIU knows this all and acts respectively :)
== How info about opened files is stored in CRIU ==
Having said that CRIU introduces several images to keep info about what files are opened by task.
=== File descriptors ===
First image is the <code>fdinfo-$id.img</code> one. This image contains info about FDT of a process. The entries have two important fields -- <code>fd</code> and <code>id</code>.
The ''fd'' is the descriptor number under which the created on restore File should be put. The ''id'' is the identifier to the File-Inode object pair in other images.
=== Files themselves ===
Fore the sake of simplicity CRIU doesn't introduce separate states for Files, Dentries and Inodes leaving it to the kernel. Instead each this triplet is treated as one object and for every Inode type (file, pipe, socket, etc.) separate image is introduced. Thus CRIU has
* reg-files.img for regular files, that are created by open() call
* unixsk.img for unix sockets
* pipes.img for pipes
* inetsk for IP sockets (both TCP and UDP)
* signalfd.img for signal fd
* etc.
In each of this files info about File and Inode of respective file is preserved. Dentry information is effectively stored there for regular files only -- the file's ''path''.
== How CRIU gets the information to dump ==